1. Field of the Invention
The present invention relates generally to monolithic vibrating sensors operating in differential mode, i.e. consisting of two vibrating members attached to the same support. Their monolithic character is favorable to reducing their fabrication cost because it avoids assembling components and operation in differential mode aims to improve measurement accuracy by reducing the impact of parasitic input magnitudes that operate in common mode on both vibrating members.
The invention relates more particularly to a device for decoupling the mechanical vibrations of the two vibrating members to prevent the measurement accuracy from being degraded when the frequencies of the two vibrations are similar to each other.
2. Description of the Prior Art
According to U.S. Pat. No. 5,962,786 in the name of the applicant, a monolithic differential vibrating accelerometer AD′ shown in FIG. 1 includes two substantially identical vibrating members TA′1 and TA′2 made from the same plate of material of uniform thickness. Each of the two vibrating members TA′1 and TA′2 is an accelerometric transducer including in particular a proof mass 21, 22, a resonator 31, 32 the resonant frequency whereof varies as a function of the acceleration applied along an axis substantially perpendicular to the plane of the plate, and a flexible frame 51, 52 connected by means of a single connecting bridge 71, 72 to the upper branch of a fixed part 1′ having an I-shaped front contour. The transducers TA′1 and TA′2 are symmetrical to each other about the central axis YY′ of the accelerometer AD′.
The fixed part 1′ is intended to be fastened to the structure of a vehicle (not shown) via a case base BA′. Accordingly, the frequency variations of each of the two accelerometric transducers TA′1 and TA′2 are representative of acceleration variations to which the vehicle is subjected. The resonator 31 is flush with one of the two faces of the plate and the resonator 32 is flush with the other face, so that an acceleration applied perpendicularly to the plane of the plate causes an increase in the resonant frequency of one of the two resonators and a substantially equal decrease in the resonant frequency of the other resonator.
The accelerometer is generally also influenced by input magnitudes other than the acceleration to be measured. When these other input magnitudes, referred to as parasitic input magnitudes, operate in common mode on both transducers, which is the case of temperature variations, for example, they cause substantially identical variations in frequency of both resonators. The output magnitude of the accelerometer AD′ being the difference between the two frequencies, it is clear that the influence of these parasitic input magnitudes may be significantly reduced and that the sensitivity of the accelerometer is substantially twice the sensitivity of the two transducers.
The function of each of the flexible frames 51, 52 is to provide a mechanical filter between the resonator 31, 32 and the fixed part 1′ and, taking a simplified view, each frame may be considered as a filtering suspension the resonant frequency whereof is significantly lower than that of the resonator. To use an electrical analogy, this type of filtering corresponds to second order low-pass filtering. The fixed part 1′ is therefore loaded very little by the vibrations of the resonator, the quality factor Q of which is therefore practically not degraded.
The mechanical design of the prior art accelerometer AD′ has drawbacks, in particular with regard to the mechanical coupling between the vibrations of the two resonators when the frequencies of the two vibrations are close to each other. This is the case in particular if, seeking to produce an accelerometer of great accuracy, the aim is to make the two transducers TA′1 and TA′2 as nearly identical as possible in order to obtain the benefit of the maximum differential effect. As a result of this, the two frequencies cross over at an acceleration value within the measurement range of the accelerometer. As the crossing point of the frequencies approaches, a phenomenon of mechanical coupling occurs that is initially reflected in an increase in measurement noise and then by perfect identity of the two frequencies for acceleration values situated in a small area called the “measurement blind area”. In this small area, the two frequencies are “locked” to each other, which prevents measurement of the acceleration. For example, for a prior art accelerometer AD′ with dimensions for measuring accelerations of up to 100 g and the accuracy whereof, with a sufficient difference between the two frequencies, is typically of the order of 10−4 g, the blind area may reach 1 g, which corresponds to the accuracy of measuring an acceleration contained in this area being degraded by a factor of 10 000.
This very large factor by which the accuracy is degraded has three essential causes: the monolithic character of the accelerometer AD′, the virtual absence of damping of the materials generally used to produce sensors of high accuracy, for example quartz or silicon, and the fact that the two connecting bridges 71 and 72 are connected directly together by the upper branch of the fixed part 1′. Accordingly, despite the mechanical filtering provided by the flexible frames 51 and 52 whereby the fixed part 1′ is loaded very little by the vibrations of the resonators 31 and 32, the very low loading of each of these vibrating members is transmitted directly to the other vibrating member. Accordingly, when the two frequencies are close to each other, the effect of this very low loading is amplified by a factor close to the quality factor Q of the resonator of said other vibrating member, which is the cause of the mechanical coupling between the two resonators.
As their monolithic character and the nature of the materials used for these sensors cannot be modified, given the advantages already cited, the only possibility remaining is to improve the mechanical design of the device supporting the two vibrating members.
Thus, in this line of thinking, to solve the problem of the mechanical coupling between the two resonators, it might appear beneficial to take inspiration from the mechanical filtering function provided by a flexible frame of the same type as those 51, 52 shown in FIG. 1 in order to produce a mechanical filter between the two connecting bridges 71 and 72, as shown in FIG. 2, in which said connecting bridges are attached to a flexible frame 5 surrounding the two accelerometric transducers TA′1 and TA′2. This would produce the monolithic differential vibrating accelerometer ADa represented in FIG. 2 for which, to obtain the maximum efficacy of the filtering action of the flexible frame 5, and in accordance with the recommendations of the patent already cited, the plane P perpendicular to the plane of the frame 5 and passing through the two connecting bridges 71 and 72 is a plane of symmetry of the accelerometer ADa. However, the shape and the disposition of the flexible frame 5 shown in FIG. 2 would not be satisfactory for a monolithic differential vibrating accelerometer because the mechanical coupling between the two resonators 31 and 32 would not be reduced sufficiently. For example, for an accelerometer ADa with dimensions for measuring accelerations of up to 100 g and the accuracy whereof, for a sufficient distance between the two frequencies, is typically of the order of 10−4 g, the blind measuring area cannot be less than 0.1 g unless the flexibility of the flexible frame 5 is greatly increased, which would reduce unacceptably the lowest natural frequency and therefore the ruggedness of the accelerometer. This result, although a significant advance over the blind area of approximately 1 g of the accelerometer ADT from FIG. 1, is therefore a very long way from enabling the accuracy of 10−4 g to be achieved throughout the measurement range of the accelerometer, and the reader will have understood that the mechanical filtering function produced by the flexible frame 5 acting as a filtered suspension is not suitable for sufficiently reducing the mechanical coupling between the two resonators and therefore the blind measuring area.